The invention relates to a heat-reflecting panel, more particularly a heat-reflecting window-pane having a neutral-color external appearance, comprising a transparent film support, an interference film of dielectric material having a refractive index &gt;2 disposed on the support, and a heat-reflecting gold film disposed on the side of the interference film remote from the transparent film support.
There is an increasing need, particularly in the glazing of buildings, for window-panes having good sunshading properties, i.e. minimum transmission of the total solar radiation combined with maximum transmission of visible light, the panes having a neutral-color external appearance and the same visual reflectivity as window-panes made of transparent glass. In some cases, only some of the rooms of a building (e.g. air-conditioned rooms for computer installations) have to be equipped with heat-reflecting panes, whereas all other rooms can be glazed with normal window glass, i.e. transparent glass. In such cases, it is desirable that the heat-reflecting panes should give the same outward visual reflection as transparent glass, so that the outside of the building presents a harmonious general appearance. The same frequently applies to buildings which are entirely equipped with the aforementioned heat-reflecting panels, if the panels have to match their surroundings (e.g. high density development in an old city).
Gold films have been found particularly serviceable as coatings for heat-reflecting panels, because they are chemically very stable and have a light transmittance below about 50%--i.e. good sunshading properties. These gold films, however, have a strongly shaded external appearance and are therefore unsuitable for the aforementioned applications. The thickness of the gold film can be reduced until the coated panels have a neutral-color external appearance and a low visible reflection, but the films are then so thin that they lose most of their sunshading properties.
In this connection an improvement is obtained by using a heat-reflecting panel of the kind in question, wherein an interference film of dielectric material having a refractive index &gt;2 is disposed between the gold film and the transparent film support (usually a silicate glass pane), because the two-film system, i.e. the interference film and the gold film can slightly extend the range of neutral-color, low visual reflections towards thicker gold films having a better sunshading effect. However, this is unsufficient for most applications. A visual reflection which is just neutral in color (when the panel is looked at from the glass side, i.e. from the outside when the window-pane is installed) is obtained if the gold film is 70 A thick.
The following technical data apply to a heat-reflecting prior art panel of the aforementioned kind, made of transparent glass with a 70 A thick gold film and a zinc sulphide interference film having a high index of refraction, disposed between the glass support and the gold film:
______________________________________ Transmission of solar radiation = 55% Reflection of solar radiation = 13% (measured from the glass side) Light tansmittance (relative to the = 66.3% sensitivity of the human eye to brightness) Visual reflection (measured from the glass = 8.2% side) ______________________________________
The last-mentioned value is completely equivalent to the visual reflection factor (8%) of a single pane of transparent glass.
In many applications, however, it is necessary to further reduce the solar-radiation transmission coefficients of the aforementioned heat-reflecting panels. In most cases, to ensure effective sunshading, the transmission coefficient has to be less than 50%. If the thickness of the gold film in the prior-art heat-reflecting panel is increased for this purpose, it is impossible to obtain a neutral-color external appearance, even if an additional change is made in the thickness of the interference film. The reason is that, in the range of interference-film thicknesses in question, the color of the interference film changes, during the increase in thickness, from reddish violet to pale green, without any neutral-color region at the transition between these colors.
The object of the present invention, therefore, is to improve the prior-art heat-reflecting panel so as to increase the thickness of the gold film and reduce the transmission of solar radiation to below about 50% while retaining a neutral-color external appearance.